Centrifugal casting process



Patented Sept. 12, 1967 3,341,641 CENTRIFUGAL CASTING PROCESS Charles D.Spencer, Morris County, N.J., assignor to Allied Chemical Corporation,New York, N.Y., a corporation of New York No Drawing. Filed Oct. 10,1963, Ser. No. 315,370 Claims. (Cl. 26472) This invention relates to acentrifugal casting process, more particularly to a centrifugal castingprocess wherein the rotation rate of the mold is varied.

When a liquid material is charged in a rotatable mold and the uprightmold is subsequently set to rotation about its vertical axis, the liquidmaterial will move upward along the walls of the mold, due to theresulting centrifugal force, as far as the walls do not converge inwardstowards the axis. The outer surface of the resulting liquid rotationalshape conforms to the walls of the mold cavity, while the interior,exposed surface of the liquid tends to assume a more or lessparaboloidal shape, the shape depending on the rotational velocity andthe surface tension of the liquid. This does not present problems aslong as the Walls of the mold cavity are also of a paraboloidal ornear-paraboloidal shape. However, if one intends to form uniform wallthickness items of a shape markedly deviating from a paraboloid, such asa straightwalled funnel or a cylinder, the control and the uniformity ofthe wall thickness becomes increasingly difiicult, or

even impossible.

Another factor in the difficulty of accomplishing uniform wallthicknesses of centrifugally cast articles, in general, results from anonuniform wetting of the mold walls by the material due to a greatvariety of possible reasons. This problem becomes especially aggravatedwhen one attempts to centrifugally form articles from materials having avery low viscosity in the casting stage, such as in situ polymerizedpolyamide resins, i.e. resins polymerized in the mold during thecentrifugal forming process. These resins polymerize and set up within agiven time; therefore, the complete deposition thereof on the mold Wallsand in the desired distribution has to be accomplished within that time,otherwise the nondeposited material will set up as one or moresolidified puddle or puddles, respectively,

If the rotational speed of the mold is fast, mold surfaces closer to thebottom tend to be starved and remote surfaces overloaded; if it isattempted to overcome this lack of uniformity by employing a lowerrotational speed, the liquid charge may remain at or near the bottom ofthe mold so that higher areas are starved; and if the mold surface isnot a continuous smooth one, but is interrupted by angles, crevices,curvatures, or protrusions, a heavy or insufiicient material buildupoccurs in these parts. Additionally, nonuniform distribution of thematerial on the walls of the mold cavity is often more pronounced in thecase of such low-viscosity materials.

In attempts to overcome the above-mentioned difiiculties, complicatedcentrifugal casting machines have been constructed to impart a compoundcentrifugal motion to the mold and the liquid charge therein, such asproposed, for instance, in United States Patent No. 2,961,703. Variousattempts made heretofore resulted only in a partial alleviation of theproblems.

It is an object of the invention to provide an improved centrifugalcasting method eliminating the above-mentioned drawbacks of the priorart methods.

According to the improved centrifugal casting process of the invention,a charge of the liquid material to be molded is inserted into astationary or rotating mold and, subsequently, depending on Whether ornot the mold is already rotating, rotation is commenced. After the moldhas accomplished acceleration to the maximum r.p.m., its rate ofrotation is alternately decreased and increased. In the alternative, themold can be alternatively rotated in a clockwise and counterclockwisedirection.

As a result of the alternating acceleration and deceleration of themold, or the alternative method, whereby the acceleration anddeceleration are coupled with a directional change, the materialcharge-d into the mold cavity Will first have a speed less than thespeed of the mold. Due to the friction between the mold and thematerial, the latter will accelerate until an equilibrium is establishedbetween the speed of the mold and that of the material. When the moldrotation is braked, the material, due to inertia, will continue rotatingby its momentum at a speed higher than the speed of the deceleratingmold, resulting in a lateral relative shearing displacement between thematerial and mold, the material thereby Washing the mold walls in alateral direction. The mold deceleration is followed by a more gradualdeceleration of the material, and the lateral slippage or displacementof the material relative to the mold diminishes in degree in time. Ifthe mold is now accelerated again, a second lateral relativedisplacement between the material and the mold walls occurs due to thefriction of the material against the wall, the second relativedisplacement being in an opposite sense as the displacement whichoccurred during the deceleration. A similar alternating relativedisplacement between the material and the mold walls can be accomplishedin both directions when the direction of the mold rotation is reversed.As a result of these lateral relative displacements, a greater wallthickness uniformity of the material of the object thus molded, alongany horizontal cross section, can be accomplished. Due to thealternating accelerations and decelera-tions and the attendant increasesand decreases in the centrifugal force acting on the material, it will,in addition to the lateral displacements referred to above, also move upand down along the mold cavity, and thereby repeatedly wash the moldWalls to permit the forming of items, the inner cross-sectional shape ofwhich markedly deviates from a paraboloid. In addition, a better andmore uniform layup or mold-wall coverage with the material will alsoresult.

In the prior art continuous rotation centrifuging techniques, slightvariations in material characteristics and temperature required aconstant readjusting and close control of the material temperature andmold rotation. By employing the alternating rotational process of thisinvention, the strict control requirements necessitated by the prior artmethods may be relaxed.

If the item to be molded is desired to be formed with stratified walls,wherein successive layers of different materials form the composite wallcross-section, the materials forming the different layers may be chargedinto the mold at successive stages. Such layers may include alternatingdeposited layers of materials having varying physical propertiescomplementing each other, alternating layers of resins, fillers,pigments, etc.

Undue material buildup in corners and crevices of the mold is avoidedbecause only a relatively thin layer of material is deposited within oneacceleration and deceleration cycle as the material shifts up and thendown along the walls, and the thin layer is well distributed in suchplaces before another layer of material is deposited thereon in asubsequent cycle.

The rate of deceleration or acceleration is limited by the possibilitythat at higher rates the liquid material may tend to splash or formwaves, and if this occurs at a point where the liquid starts to solidfyor polymerize, the irregularities may be frozen into the final moldedbody shape. This limiting rate has to be determined individually case ofvery fluid.

for the particular material used, surface tension and mold shape.

The rotational speeds employed will greatly vary from one material andmold shape to another, but in the materials, such as anionicallypolymthe rotational speed usually always in view of its viscosity,

erized caprolactam, exceeds 50 r.p.m.

In the following, by way of example, an embodiment of the process, ofthe inventionis given, as applied to centrifugal polymerization ofliquid lactams while casting objects thereof, thel-a-ctams generallyhaving a viscosity of about 10 centipoises in the monomer state;however, it is to be understood that the process of the invention isalso applicableto the .centrifugal casting of other materials which havedifferent viscosities, The minor adjustments of the processingparameters for any given material may be determined by routineexperimentation.

Example An aluminum vessel 11 inches in diameter and 7 inches deep andhaving a flat bottom joined by a gentle curve to the slightly tapered,almost cylindrical sidewall was mounted in an electric oven as a moldfor rotation about. its axis. The mold vessel was covered and sealedexcept for a 1-inch hole in the center for purging the mold cavity withan inert gas and for introducing liquid resin therethrough. The mold Waspreheated to 175 C. and the cavity purged with dry nitrogen.

With the mold rotating at 500 r.p.m., 250 g. of a mix ture consisting of247 g. caprolactam, 2% g. lithium caproylimi-de, erization catalyst andpromotor at 100 C. was charged through the filler. opening. Materialsofthis type are described in greater detail, for instance, in BelgianPatent No. 623,840. During the handling and pouring, the mixture wasblanketed vwith dry nitrogen to prevent its exposure to atmosphericmoisture and oxygen.

As soonas the mixture was fully charged into the mold, the mold rotationwas braked from the original 500 r.p.m. to a full stop in 5 seconds.After a -secondrest the mold'was accelerated again in 5 seconds to 500r.p.m. by a torque motor driving the mold through a belt. As soon as themold reached the full speed, it was again decelerated, and this20-second cycle was repeated for. a period of 5 minutes.

A second 250 g. charge of then poured into the mold rotating at 500r.p.m. and the above 5-minute acceleration-deceleration cycling wasrepeated. After the mold was stopped, it was maintained 7 at 175 C. forminutes. The resulting saucepan-shaped item had excellentcircumferential wall thickness uniformity; the vertical wall thicknessdistribution was also satisfactory; The two successive charges gave across-section that appeared as if only one charge had been made.

It is to be understood that the invention is disclosed by way of anexample and, depending on production requirements and materialcharacteristics, a number of and 0.63 g. triphenoXy-s-triazine aspolym-' modifications may be employed. Different materials and differentproduction rates would require different processing temperatures anddurations, while some materials, such as thermoplasticresins may alsorequire cooling during the process. Depending on the viscositycharacteristics of the material to be molded, a variety of acceler-ationand deceleration rates as well as rotation times may beused. The processof the invention is not limited to centrifuging only about a verticalmold axis, the process is also applicable to the centrifugal casting ofarticles in which the walls above the bottom plane converge inward atany point. In this case the rotational axis of the mold has to beinclined from the vertical at some point during In view of theforegoing, the scope the rotational cycle. of the invention is toclaims.

I claim: 1. In a centrifugal casting process wherein a mold is chargedwith a materialto a molded, and the material is converted therein fromafluid liquidv to a solid phase,

be interpreted from the appended the improvement which comprises arepeated cycle of j rotating said mold about its vertical 'aXis to.adesired rotational speed of at least 50 r.p.m. and quickly braking saidrotating moldwhen it attains said desired rotational speed, each cyclecomprising about 10 seconds of rotation in which, the mold is notfollowed by about 10 seconds rotated.

2. The process of claim 1 wherein the direction of rotation is reversedabout each braking.

3. The process of claim 1 wherein the desired speed of rotation-is inthe range from 50 to about 500 r.p.m.

4. An improved centrifugal casting process which comprises charging amold with a mixture, consisting the mixture at 100 C. was ipolymerization promotor followed by essentially of caprolactam, ananionic catalyst and a the repeated cycle of rotating the mold about itsvertical axis to a about 500' r.p.m., quickly braking the rotationwithin -a periodof about .5 seconds when it attains the desiredrotational speed and permitting the mold to'remain at least for about 10seconds prior to repeating the cycle.

5. The process of claim rotation is reversed for'each succeeding cycle.

References'Cited UNITED STATES PATENTS ROBERT F. WHITE, S. I. LANDSMAN,Assistant Examin n Primary Examiner.

speed of 4 wherein the direction of

1. IN A CENTRIFUGAL CASTING PROCESS WHEREIN A MOLD IS CHARGED WITH AMATERIAL TO A MOLDED, AND THE MATERIAL IS CONVERTED THEREIN FROM A FLUIDLIQUID TO A SOLID PHASE, THE IMPROVEMENT WHICH COMPRISES A REPEATEDCYCLE OF ROTATING SAID MOLD ABOUT ITS VERTICAL AXIS TO A DESIREDROTATIONAL SPEED OF AT LEAST 50 R.P.M. AND QUICKLY BRAKING SAID ROTATINGMOLD WHEN IT ATTAINS SAID DESIRED ROTATIONAL SPEED, EACH CYCLECOMPRISING ABOUT 10 SECONDS OF ROTATION FOLLOWED BY ABOUT 10 SECONDS INWHICH THE MOLD IS NOT ROTATED.